This invention relates generally to voltage controlled oscillator circuits. It is particularly directed to the provision of a voltage controlled oscillator for use in low voltage devices such as electronic timepieces, to produce voltage pulses having a frequency and duty cycle which varies in proportion to the applied voltage.
VCO circuits produce output signals whose frequency varies as a function of an input control voltage. VCOs are useful in systems employing phase locked loops (PLL) as well as in any system where a variable frequency output signal is required as a function of the input signal. A VCO circuit employing cascaded inverters connected in a ring configuration to form a ring oscillator is described, for example, in U.S. Pat. No. 3,931,588 entitled "Voltage Controlled Oscillator Utilizing Field Effect Transistors" issued Jan. 6, 1976 to W. S. Gehweiler. In this reference the frequency of the ring oscillator is varied, over the relatively high operating range of approximately 6 MHZ to 21 MHZ, by varying the operating voltage applied across at least one of the inverting stages of the oscillator.
A VCO circuit employing cascaded inverters to form an astable multivibrator is described, for example, in U.S. Pat. No. 3,878,482 entitled "Wide Band Voltage Controlled Oscillator Utilizing Complementary Metal Oxide Semiconductors Integrated Circuits And A Constant Current MOS Field Effect Transistor" issued to L. F. Schowe, Jr. In this reference, the operating current and voltage is applied to a complementary inverter where a transistor of P conductivity type functions as a current source. As the conduction of the P type transistor is varied more or less current flows into the inverter.
Other prior art patents of interest include U.S. Pat. Nos. 4,156,204 issued May 22, 1979 to Robert N. Hargis; 3,911,378 issued Oct. 7, 1975 to James E. Buchanan; 4,072,910 issued Feb. 7, 1978 to Andrew Gordon Francis Dingwall et al.; 4,146,849 issued Mar. 27, 1979 to Kouichirou Satou; and 3,919,625 issued Nov. 11, 1975 to Troyce D. Barton.
In contrast to the prior art, the present invention provides a VCO circuit which varies both frequency and duty cycle as a function of the applied voltage, provides an output voltage/current source which varies inversely to the applied (input) voltage and has particular application for low voltage and low frequency operation.